The carbon in the atmosphere, ocean, surface life, and other shallow, near surface reservoirs accounts for only about 10% of Earth’s carbon. Where is the other 90%? What is it doing? Does it matter? The Deep Carbon Observatory (DCO), an ambitious 10-year (2009-2019) program of exploration and experimentation, pursues the mysterious 90% while building a new scientific field with a network of scientists from more than 40 countries. Recent results from DCO researchers are filling in the global carbon puzzle with findings that extend our understanding of the origins and limits of life on Earth, what erupts from volcanoes and leaks from sea floors, and what descends back into Earth’s great depths.